Tag Archives: Corrosion

Industrial processes across the world require pumps to operate efficiently.

The latest pump designs and coating technologies offer significant improvements and by lessening the effects of corrosion and erosion, productivity can be enhanced whilst running costs are reduced.

Research into the processes that degrade pump performance is continuously carried out in the industry, and is in fact now being matched by the development of effective protective coatings. By gaining a better understanding of both the pumping process and the factors that effect it, significant improvements can be made to maintenance strategies.

Affected Applications

Almost every industrial process involving liquids will require a pump. From deep sea oil and gas to DNA sequencing, pumps are required to perform a vast range of tasks, daily. However, regardless of the design or size of the pump, central to every application is reliability and efficiency.

Minimising down time and running costs is essential to business.

For those working with large industrial pumps, often operating in harsh environmental conditions maintaining pump performance, with corrosion and erosion being a continuous threat, can be a real challenge.

In most cases it is necessary to implement a cost effective pump refurbishment programme.

Corrosion

Often defined as a chemical reaction between the component surface and the reacting fluid passing through a pump. In general a distinction is drawn between general or uniform corrosion and localised corrosion. Non-stainless materials suffer mainly from uniform corrosion whereas metals forming oxide layers that adhere to and passivate the surface are prone to localised corrosion.

Flow Accelerated Corrosion

Flow accelerated corrosion (FAC) describes the removal of the protective oxide layer on a metal. The speed of this process is affected by the oxygen content, the flow velocity and, to some extent, the chloride content. The formation of a calcareous layer due to high carbonate hardness of the water reduces or even prevents FAC. The influence of oxygen can be seen in the following example: water with an oxygen content of less than 20 ppb (parts per billion) and a flow velocity around 15 m/s will typically see a corrosion around 0.01 mm/year. However, increased oxygen content can see the corrosion rate rise to several mm/year, which will present a significant challenge to the process.

Erosion

Pumps that are used to transfer fluids containing abrasive substances, such as sand, can experience significant levels of erosion, especially in areas with high flow velocities. This can be seen frequently in the gas and oil industry, where injection pumps are employed to force water back into the oil field and therefore maintain pressure which is needed to lift the oil to the surface. The sand particles act as an abrasive and the high working pressures only serve to compound the issue.

In operating conditions where both erosion and corrosion are present, the degradation mechanism can become very complex and depends on the type of substrate and the fluid chemistry.

Corrosion may create oxide layers with low adherence to the substrate which is prone to erosion, or erosion may damage the passive layer, leading to an activation of the surface which accelerates corrosion. In this case surface protection regimes are often the best and sole option.

Cavitation

Most commonly seen on the pump impeller, cavitation is caused by a pressure difference, either on the pump body or the impeller. A sudden pressure drop in fluid causes the liquid to flash to vapour when the the local pressure falls below the saturation pressure for the fluid being pumped. Any vapour bubbles formed by the pressure drop are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region where the local pressure is greater than saturation pressure, the vapour bubbles abruptly collapse, creating a shock wave that, over time, can cause significant damage to the impeller and/or pump housing.

In most cases it is better to prevent corrosion and erosion rather than trying to reduce the effects on the pumping equipment.

Consider the following:

Increase the suction head

Lower the fluid temperature

Decrease the Net Positive Suction Head Required (NPSHR) for situations where cavitation is unavoidable or the pumping system suffers from internal re-circulation or excessive turbulence, it may be necessary to review the pump design or minimise the potential for damage using a bespoke coating system